This question is no doubt will be listed on the top for non-native Chinese parents who have a child growing up in the family. In my opinion, for a foreign language or 2nd language acquisition for your child, I would say never say it is too early or too late to start comparing with the native language acquisition path.
When non-native Chinese parents ask me “what is the best age to learn Mandarin”, I would ask them another question instead to give the answer, like what level of Mandarin do you want your children to achieve? Do you want to just nurture your child some Mandarin Rhythm? Do you want your child to gain a fluent-like speaking Chinese ability like Chinese native children?
The answers from parents will reveal the learning goal for your children. It can make clear on what a learning plan or program is the best for the children at their current stage with their family and their neighborhood background. Mandarin fluency or Mandarin proficiency, which will you consider more?
Within your determination or plan, your children can start from a small goal and head for a bigger goal with a clear time span frame for getting into the Chinese learning track smoothly. To consider the learning path for Mandarin fluency or Mandarin native-like, of course, the best age for your child to learn Mandarin is earlier the better. In the meantime please do start to consider an appropriate Chinese program and the dedicated teacher or tutor who actually can protect your dear children in an appropriate Chinese learning journey.